On Thu, 2 Sep 2010 02:41:15 +0100, "Arfa Daily"
wrote:
The soldering under the BGAs letting go, is the commonest problem with both
the Sony PS3 ( "yellow light of death") and the X-Box 360 ("red ring of
death")
Those names for the conditions refer to the behaviour of the front panel
indicator LEDs when the faults that result, show themselves.
I am quite convinced that the slow running of the fan at idle, is a major
contributory factor in the failing of the BGA soldering.
On the other foot, I suspect that a high air flow fan will make it
worse. The problem is NOT that the BGA is flexing with increasing
temperatures. It's that the PCB underneath the BGA is flexing.
Stabilizing the temperature of the BGA is probably useful, but unless
the PCB is also stabilized, it will bend, bulge, buckle, twist, or
otherwise go through various contortions trying deal with the
temperature difference between the BGA and the PCB. If the
differential temperature is large enough, the PCB may bulge enough to
tear way from the BGA. Again, the BGA does not move, the PCB does.
Now, add a high air flow fan into the picture and we have a larger
temperature differential. The air flow will probably do a fair job of
cooling the PCB because of the comparatively smaller mass of the PCB.
The thermal conductivity of G10/FR4 isn't all that wonderful,
resulting in a localized hot spot. With a larger difference between
the BGA area and the surrounding PCB, the result is a larger PCB bulge
with PCB air cooling. I've seen PCB's (usually motherboards) with
permanent bulges under BGA's from this effect.
For entertainment, take any PCB, heat it in the middle with a heat
gun, and watch the bulge form. It's that bulge that's ripping the
BGA's apart. Extra credit to laptop manufacturers, that add heat
sinks to the BGA, and then mechanically connects the heat sink to the
frame. When the board bends, it will literally tear the BGA off the
PCB, since the heat sink can't move with the board.
In the instructions for hot air reflowing of BGA's, there's usually a
section on pre-heating and slow cool down of the PCB. The idea is to
not tear the BGA ball apart from differential thermal expansion
between the large thermal mass of the BGA and the comparatively
smaller mass of the PCB. It's exactly like moving a solder connection
while it's cooling. You get a "cold" solder joint.
Incidentally, I once designed a 150 watt 2-30Mhz HF power amplifier.
After about a year of normal use, we started seeing failures caused by
the power transistor screws coming loose. Suspecting cold flow, I
worked on improving the grip with stainless inserts. This worked,
but now produced failures in the ceramic case power transistors. The
clue was when a PA module arrived with all the ceramic lids popped off
the transistors, but still working. Weird(tm).
After a dozen bad guesses, I determined that PCB expansion and
contraction was initially causing the loose screws. When the screws
were properly secured, the next weakest link was ripping the leads out
of the power transistor case, causing the glued lid to pop off. The
problem was solved by slightly pre-bending the power transistor leads
in a fixture so that PCB thermal expansion would be absorbed by the
bends. I still do this even on TO220 packages, which can have the
same problem. Too bad it can't be done with BGA packages.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558